Americans celebrates Independence Day amid heightened security following attempted car bombings in Britain, and thousands of immigrants became US citizens at ceremonies around the country.

Americans celebrated Independence Day on Wednesday amid heightened security following attempted car bombings in Britain, and thousands of immigrants became US citizens at ceremonies around the country.

In Washington DC, security was increased on the National Mall as organizers sought to reassure visitors.

Hundreds of emergency responders from about 20 law enforcement agencies were on duty, authorities said. A police helicopter was to monitor crowds from above, and officers were urged to be on alert for vehicles that looked suspicious, for instance with protruding wires or an unusual smell.

As with past July 4 festivities since the September 11 terrorist attacks, the mall was fenced off and visitors will be required to pass through security checkpoints.

Independence Day festivities in Washington include a parade on Constitution Avenue, a concert by the National Symphony Orchestra on the West Lawn of the Capitol and a 20-minute fireworks show.

In New York, the Fourth of July fireworks display billed as the country's biggest introduces a pyrotechnic novelty: exploding shells aimed at the water, not the sky.

The so-called nautical shells are supposed to explode on the surface of the East River, remaining illuminated for a few seconds before fading out, said Robin Hall, executive producer of the Macy's Fourth of July display.

In Iowa, Democratic presidential candidate Hillary Rodham Clinton walked hand in hand with her husband Bill in a sunny, postcard-perfect holiday parade in the north-central lakeside town of Clear Lake.